1. Field of the Invention
This invention relates in general to a supplementary device for use in telephone communications which carries out a given operation in response to the reception of call signals sent out over a telephone line, and more particularly to a variable impedance, non-interfering, automatic telephone answering device.
2. Description of the Prior Art
There have been proposed supplementary devices such as an automatic telephone device, a remote bell device, etc. which are connected in parallel to a telephone handset across a telephone line. Generally, when such a supplementary device is connected to a telephone line the associated telephone handset is adversely affected, which results in the degradation or complete disruption of the communicating function. In order to avoid such problems various technical standards with respect to supplementary devices have been established by various regulatory agencies. One such technical standard requires that in an automatic telephone system or communicating battery system, the impedance value of the supplementary device during the reception of incoming signals must be more than 2K.OMEGA. per circuit at frequencies under 15Hz (ring up signal or call signal frequency).
As a result of this standard supplementary devices for domestic use are designed to have an impedance of 10K.OMEGA. to provide an adequate safety margin based on the supposition that (1) the supplementary device will be connected in parallel to a telephone handset having an impedance of about 10K.OMEGA., (2) a plurality of supplementary devices will be connected in parallel across a telephone line, or (3) a parallel connection of different supplementary devices will be connected in parallel to a telephone handset. A further design criteria centers around the fact that the supplementary device may use the call signal as a source of its electric power.
An automatic telephone answering device, which is one type of supplementary device, receives call signals during the absence of a party, automatically plays back a prerecorded message to the caller, and then records a message from the caller. Such automatic telephone answering devices are constructed such that the call signals are fed to an introduction means whose output drives the control circuit of a tape recording unit. The call signal introduction means are designed to have an input impedance of 10K.OMEGA. in order to have no adverse influences on the associated communication system.
However, conventional supplementary devices are only designed to have relatively low impedances during the reception of call signals of 16-20Hz at a voltage of 70-100V r.m.s. Therefore, the impedance drops almost exponentially in the range of sound signal frequencies of 300-3400Hz and busy tone frequencies of 400Hz appearing over telephone lines.
In addition, when the sound signal frequency is relatively high the impedance value decreases to 1-2K.OMEGA.. As a result, when the supplementary device and a telephone handset are connected in parallel across a telephone line, and when a called party is answering the caller over the telephone, the input impedance of the supplementary device varies in response to the change of the sound signal frequencies. This varies the load impedance seen by the sound signal, which results in voice distortion, a decrease in message quality, such as voice articulation, and excessive sound signal loss at high frequencies.